State Department of Environmental Protection officials announced Friday they have ordered the Dan A. Hughes Co. to cease operations at a second South Florida well until experts can analyze whether the violation at the first well spread pollution in the aquifer.

DEP officials did not give details about why they were taking this step more than a week after shutting down operations at the well where the violation occurred. DEP spokeswoman Dee Ann Miller said the agency "has been in dialogue with the Dan A. Hughes Co. regarding their plans and permits ... and this is part of that ongoing process."

However, the announcement followed a call Thursday by U.S. Sen. Bill Nelson for the U.S. Environmental Protection Agency to open an investigation into what happened.

Hughes spokesman David Blackmon issued a terse statement that said the shutdown was something the company had agreed to after negotiations with DEP.

"Protection of human life and the environment are our company's highest priorities," he said in the statement. "We commend the DEP for its diligent efforts to ... reach an approach to this matter that satisfies the needs of all stakeholders."

Hughes' drilling operations have sparked controversy among Collier County residents, who went so far as to stage a protest march on Gov. Rick Scott's Naples home. The controversy began when a Hughes contractor contacted Golden Gate Estates residents about their plans for an evacuation should something go wrong with the well being drilled less than 1,000 feet from their homes. That was the first they'd heard of it.

That permit was approved by DEP but challenged in court. Meanwhile a new controversy has swirled around one of the company's other wells in that region.

A 12-page consent order, dated April 8, says DEP officials became concerned about an operation that the Texas company launched without DEP permission in late December 2013. They told the company to shut it down, but it kept going for another day.

The company was injecting acid deep underground to fracture the limestone, then injecting a mix of sand and chemical gel under pressure, to prop open the new fractures and let the oil flow out. That's called using a "proppant."

Although the process is similar to fracking, Blackmon called it an "acid stimulation treatment," which he said is common in Florida. However, Miller of the DEP said no one has ever used a proppant in Florida before. The DEP order requires Hughes to install monitoring wells to check on whether any pollution is spreading through the aquifer, although Blackmon said the chemicals were injected thousands of feet below the drinking water supply.

Until the well results are analyzed by independent experts, Hughes has to shut down all other "new operations," DEP said Friday. Hughes has six other locations, but the only one fitting the DEP's description is a well near Immokalee, Miller said.

Craig Pittman can be reached at craig@tampabay.com or follow him on Twitter via @craigtimes.